Fuel cell systems have been proposed for use in electrical vehicular power plants to replace internal combustion engines. Such systems typically include a proton exchange membrane (PEM) type fuel cell in which hydrogen is supplied as the fuel to the anode and oxygen is supplied as the oxidant to the cathode of the fuel cell. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. These MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO) for effective operation. A plurality of individual cells are commonly arranged in series together to form a fuel cell stack. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
For vehicular applications, it may be desirable to use a liquid fuel such as a liquid hydrocarbon (e.g., methanol, ethanol or gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor typically contains multiple reactors wherein the fuel reacts with steam and air to yield a reformate gas comprising primarily hydrogen and carbon dioxide. A primary reactor is utilized to dissociate the hydrocarbon fuel into hydrogen, carbon dioxide, carbon monoxide, water and methane. Secondary or CO reduction reactors are used to reduce the CO levels in the reformate stream.
The feasibility of using a fuel cell system as a power source can depend on whether the size of the system is appropriate for a particular use. This is particularly so in vehicular applications, where the mass and volume of a vehicle directly influence its fuel efficiency and speed. Another factor affecting fuel cell system performance is the amount of water available within the system for input to reactions requiring water as a reactant. It is desirable to maintain a water-neutral fuel cell system and at the same time to provide enough water to support efficient fuel cell performance. Thus, steam to carbon (S/C) ratio is an important consideration in fuel processor design.
Fuel processors for a fuel cell system have heretofore been designed in the following manner. A fuel processor is typically designed with primary emphasis on operational efficiency at a maximum power operating point and secondary emphasis on other specifications such as turndown ratio, acceptable start-up duration, and transient performance. A maximum-power, steady-state flow mechanization is used to predict the carbon monoxide concentration of the primary reactor. The CO reduction reactors are then sized to reduce the carbon monoxide levels to a sufficiently high quality for the fuel cell stack at the maximum power operating point.
Since fuel processors for automotive applications are in an early state of development, no alternate design methods or strategies currently are known that utilize existing technology to optimize the size (i.e., mass and volume) of the fuel processor. Although a maximum-power steady-state flow mechanization currently is analyzed in the fuel processor design process, it would be desirable also to analyze fuel processor operation at a plurality of power levels and to configure a fuel processor for efficient performance for the power levels at which it operates most of the time.